One common type of electronic circuit board is Surface Mounting Technology (SMT). As opposed to the most basic form of traditional through board mounting, in which electronic components such as IC's, transistors, and resistors are positioned on the top surface of a circuit board with their connection leads passed through holes in the board and soldered to conducting pathways on the bottom surface of the board, in the most basic form of SMT, conducting pathways run on the top surface of the circuit board to interconnect the electronic components, which are positioned on the top surface of the board with their connection leads soldered to the pathways on the top surface. SMT circuit boards are not limited to having all of their conductive pathways on the top surface of the board however. Two sided boards having pathways on the top and bottom surfaces and multi layered boards having pathways laminated within the board are also common forms of SMT. The pathways in differing layers may be electrically connected by conductive via's, which are most basically described as electrical connections that pass through holes drilled through the board from one pathway to another.
For certain components, the soldering of the leads to the lead pads on the pathways provides both the electrical and mechanical fastening of the component to the board. For other components, such as those with more significant mass or height, or those which generate heat during use, like IC's and high power transistors, Surface Mounting Pads (SMT pads) are formed on the top surface of the board, to which the component's base may be affixed. SMT Pads are planar solder coated or solder ready surfaces formed, usually by reverse etching, on and integral with the top surface of the board just under the intended location of the component's body.
SMT pads provide a structural foundation for securely soldering the component to the board to avoid reliance for mechanical affixation solely on the soldering of the leads. This reduces stress on the lead connections and improves connection reliability, particularly for components of more significant mass or higher profile which are more susceptible to lead connection stress. Additionally, SMT pads may be thermal sinks which conduct damaging heat from energy consuming and heat generating components such as IC's, thereby facilitating faster operating speeds and higher power consumption.
SMT pads may further provide an electrical connection, most often a grounding, of the component base to the circuit. Surface mounting electrical components intended for affixation to SMT Pads generally have a metallic base or base plating which may be physically integrated with the component body and which may be thermally connected to the heat generating elements of the component. This metallic base may have a flat bottom surface. The metallic base may be made of or plated with a solder compatible material, such as copper or tin.
SMT pads are generally sized and shaped according to standardized footprints that match the base surface size and shape of the components that will be mounted on the pad. This provides the most effective mechanical affixation, and the most efficient thermal and electrical communication. A coating of flux may be layered between the SMT Pad and the component base prior to positioning the component on the board, with the goal of facilitating a complete and void free solder bond between the SMT Pad and base during soldering. When the component is properly positioned on the top surface of the board, each lead is positioned atop its related lead connection pad on the board, and the metallic base is positioned atop and aligned with the SMT Pad. The flat bottom surface of the base preferably lies directly on and against the flat surface of the SMT pad. A common technique for soldering the components to the board is to pass the board, with all components in place, through an oven to melt the solder. This preferably results not only in the electrical solder connection of the leads to the pathways, but also in the molten solder completely filling the space between the component bases and their associated SMT Pads, hopefully expelling any gas there from.
Due to the plane to plane relationship between the component base and SMT Pad, a common manufacturing problem during soldering is the creation or entrapment of gases within the thin space between the components and pads. Air existing in these junctions prior to soldering due to any non-planarity of either surface may become entrapped, say for instance when the perimeter of the base is soldered before soldering is complete. Off-gases from the soldering process or from oxidation of impurities in the solder may also develop in the junction.
Reduction of mechanical strength of the base to SMT Pad solder joint is one problem resulting from such gas voids. Additionally, the reduction in thermal and electrical conductivity between the base and the SMT pad can create significant performance and reliability problems for the associated component and the entire circuit, which is difficult to initially detect by any known inspection technique. Also, entrapped gases may expand as the component heats up during use, thereby causing pressure between the component and circuit board which may lead to numerous other types of component and circuit failures which are difficult to detect during initial testing. And entrapped gases may cause catastrophic failures in circuit boards used in high or low pressure environments, such as underwater or in deep space.